Digital Filter Usage

Something is wrong with your premise / setup. And I do not understand some details. BUT, a 3kHz sampling filter of 60Hz, takes it well beyond Nyquist. Much beyond it, unnecessarily, unless harmonics needed to be reproduced properly. You forgot to mention any of it.

A properly designed filter for such a purpose supposed to carry all components from DC up to whatever frequency component is desired.

Any lower frequency corner??!?? What for? Explain!

hello leveles thank u for ur response

i have a piece of code written for a ups which is supposed to calculate the mean square value of the input signal, the comment of the code contains the transfer function which i mentioned and the value of 'a' is as stated in my question, my task is to identify the spec of the digital filter used using the transfer function and the coefficient 'a' ,if u can see in my calculations, u can see that i used the magnitude value of H(e^jw) to compute the maximum magnitude from where i calculated the -3db point and found out the corresponding frequency since the input to the digital filter is a squared value of the raw ADC output i think this filter is used to remove the quantization noise (low cutoff .23Hz,7.5Hz).

now if u could answer to my queries in the numerical fasion that i asked them it would be very help ful,

hope the additional info that i provided is helpful....

Try referring to the following :-

http://www.bores.com/courses/intro/iir/5_eq.htm

I got to that by using a search for "digital filter" in "Wikipedia"

http://en.wikipedia.org

[which has a number of free filter design software links] - which got me to :-

http://en.wikipedia.org/wiki/Digital_filter#Types_of_digital_filters

-- and then the "see also..." part of that had a link to "infinite impulse response" - which led to the external link listed first.

I guess the basic formula you give has to be modified for sample rate.

Also, a routine to calculate the rms value of a real mains frequency voltage would have to include the major harmonics like 3rd, 5th, 7th to have good accuracy - hence a low-pass filter with passband to say 50 Hz x 7 = 350 Hz at -3 dB.

A low pass filter leaving just the fundamental (e.g. -3 dB at 50 Hz) would make rms processing pointless, because the output would be a sine form and using the mean value would be as effective for a control loop as rms.

Hallo Bharat!

It seems to me, that you misunderstood the very nature of this website we are conversing on. Here, you ask whatever you want to, and I or somebody else answers as we see fit.

In this particular case, I fail to see sense in the question. Let me explain, why. In a negative feedback loop a DC value and many higher frequency components have to be carried. There are amplitude and phase requirements, hence the requirement for some specific filter. BUT, DC sets the operating point. The rest is valuable enhancement only: like fast response, low noise.

You may have an exquisitely digitized system. But, in any synchronous digitized system, THERE IS NO LOWER BANDPASS EDGE, NONE. I cannot comprehend the need for one. Digitization noise at a low frequency? What the heck you are talking about in the first place?? I have never met one. Explain, fully now.